Bell, Suzanne S., author
Libraries Unlimited, 2015
Buku Teks
Gillham, Bill, author
Continuum , 2000
Buku Teks
Yin, Robert K., author
Sage, 2003
Buku Teks
edited by Michael J.M. Martin
Wiley-Interscience , 1971
Buku Teks
Hall, Robert B.
Science Research Council , 1947
Buku Teks